Binary-Phase Acetonitrile and Water Aerosols: Infrared Studies and Theoretical Simulation at Titan Atmosphere Conditions

Acetonitrile (CH₃CN) and water (H₂O) ice particles were generated within a collisional cooling cell coupled to the Australian Synchrotron light source. The evolution of the aerosols was tracked by infrared spectroscopy compiled over the 4000–50 cm^–1 region. Gas pressure and temperature conditions were varied to replicate the lower altitudes of the Titan atmosphere allowing for comparison to far-infrared features detected by the Cassini–Huygens spacecraft. The experimental spectra show that CH₃CN and H₂O particles are microheterogeneous in composition and spherical in shape. CH₃CN lattice bands display temperature-dependent shifts in frequency, implying that pure β-phase is present in the mixed particles. In addition, a red shift identified for the CN fundamental stretching mode indicates dipole–dipole and π-electron side-directed hydrogen bond coupling between segregated CH₃CN and H₂O phases exclusively at the grain interface. Discrete dipole approximation theory was implemented to evaluate various cluster architectures where segregated domains of pure CH₃CN and H₂O ices provided the best fit to experiment; confirming the infrared findings. Otherwise, simulations of competing architectures, such as core–shell and cubic shaped particles, did not provide convincing comparison to the aerosol spectra. We conclude that the far-infrared profiles for mixed CH₃CN–H₂O systems do not present as likely carriers for the unassigned 220 cm^–1 “haystack” feature that has been identified in Titan’s lower atmosphere.